i'm sorry...what's decimation?

hi!
i'm sorry what is decimation?

yesterday, i found a block diagram of digital signal processing for
demodulate AM signal...
and there is a decimation process after the balance modulator...
i don't understand the function of decimation process...

please, help me...
thank you...

-c1910- 

maybe this link helps you http://www.prosig.com/signal-processing/DataDecimation.html